A conventional printing device is constituted, for some types of franking machines, by a cylinder bearing printable characters engraved in relief on its periphery. Said cylinder rotates on its shaft when printing movement is ordered. During the first part of its rotation, the characters pass in front of an inking device which deposits a layer of ink on their upstanding surfaces.
Actual printing takes place when the characters covered in ink come into contact with the document to be printed. Said contact is provided and its pressure controlled by a pressure roll which presses the document against the printing cylinder and simultaneously serves to move the document to be printed.
The devices which feed the documents to the printer and which provide the contact pressure are generally constituted by rubber rolls or conveyor belts.
These rolls or conveyor belts are held in position by mechanical components which position them in space relative to the printing cylinder, in particular, whose shaft is stationary. To frank documents of any thickness, these mechanical supports must allow the rolls or conveyor belts to move away a distance equal to the thickness of the document while keeping the contact pressure constant. Up till now, the contact pressure on the printing cylinder has been adjusted by imparting to the pressure roll a movement which is only parallel to the printing cylinder.
Further, these rolls or belts must also feed the documents into the franking machine and consequently they must be made to rotate by a drive unit.
Said drive function is therefore complicated by the thickness corrector function since a rotary movement must be provided for a unit whose shaft is free to rotate.
Further, the documents to be printed not only vary in thickness from one to another but may also have poor uniformity of thickness and in particular be thicker at one end than at the other.
Such types of defect are not usually properly corrected in franking machines and lead to a printing contact pressure against the characters which is higher at one end of the cylinder than at the other. It follows that the characters make a greater impression where the pressure is highest.
Preferred embodiments of the present invention remedy these drawbacks. Indeed, they make it possible to define a device which takes into account all the requirements for performing the functions necessary for feeding documents to be printed into a franking machine and for generating printing pressure, in particular:
feeding the documents into the franking machine at the right speed; PA1 automatic adaptation to variation in thickness; PA1 automatic adaptation to uneven thickness; and PA1 creation and maintenance of a constant printing pressure.